The present invention relates to a variable gain circuit whose gain is externally controllable, and particularly, to a variable gain circuit operable at a low supply voltage.
A configuration using a Gilbert multiplier is common as a variable gain circuit. An example of a variable gain circuit according to a related art is shown in FIG. 3. In FIG. 3, the related-art, variable gain circuit has a differential amplifier circuit 101, two current divider circuits 102 and 103, a control voltage (Vc) generating source 104, and a bias voltage (Vb) generating source 105.
The differential amplifier circuit 101 is formed by: NPN-type differential pair transistors Q101 and Q102; an emitter resistance R101 connected between emitter electrodes of the differential pair transistors Q101 and Q102; and constant current sources I101 and I102 connected between a ground and the emitter electrodes of the differential pair transistors Q101 and Q102, respectively. Base electrodes of the differential pair transistors Q101 and Q102 are connected to circuit input terminals 106 and 107, respectively.
One current divider circuit 102 has a differential circuit configuration formed by NPN-type differential pair transistors Q103 and Q104, each having an emitter electrode commonly connected to a collector electrode of the transistor Q101, and a resistance R102 connected between a collector electrode of one transistor Q103 and a power supply Vcc. A collector electrode of the other transistor Q104 is connected directly to the power supply Vcc.
The other current divider circuit 103 has a differential circuit configuration formed by NPN-type differential pair transistors Q105 and Q106, each having an emitter electrode commonly connected to a collector electrode of the transistor Q102, and a resistance R103 connected between a collector electrode of one transistor Q105 and the power supply Vcc. A collector electrode of the other transistor Q106 is connected directly to the power supply Vcc.
The collector electrodes of the transistors Q103 and Q105 in the current divider circuits 102 and 103 are connected to circuit output terminals 108 and 109, respectively. Base electrodes of the transistors Q104 and Q106 are commonly connected to a positive electrode side of the control voltage generating source 104, whereas base electrodes of the transistors Q103 and Q105 are commonly connected to a negative electrode side of the control voltage generating source 104. A positive electrode side of the bias voltage generating source 105 is connected to the base electrodes of the transistors Q103 and Q105, while a negative electrode side of the bias voltage generating source 105 is connected to the ground.
In the thus formed variable gain circuit, letting Ic1 and Ic2 be collector currents of the transistors Q103 and Q104 and Vc be a control voltage of the control voltage generating source 104,
Ic2/Ic1=exp(Vc/Vt)xe2x80x83xe2x80x83(1)
where Vt=kT/q, k being the Boltzmann constant, T being the absolute temperature, and q being the amount of electron charge.
Letting IA be a current value of the constant current source I101, since Ic1+Ic2=IA,                                                                         Ic1                /                IA                            =                              Ic1                /                                  (                                      Ic1                    +                    Ic2                                    )                                                                                                        =                              1                /                                  {                                      1                    +                                          (                                              Ic2                        /                        Ic1                                            )                                                        }                                                                                        (        2        )            
Hence, when substituting the equation (1) into the equation (2),
Ic1/IA=1/{1+(exp(Vc/Vt))}xe2x80x83xe2x80x83(3)
Letting vi be an input voltage, vo be an output voltage, RA be a resistance value of the resistance R101, and RB be resistance values of the resistances R102 and R103, a gain Av of the variable gain circuit according to the related art is given by:                     Av        =                              vo            vi                    =                      2            ⁢                                          RB                RA                            ·                              1                                  1                  +                                      exp                    ⁢                                          xe2x80x83                                        ⁢                                          Vc                      Vt                                                                                                                              (        4        )            
As is clear from the equation (4), the gain Av can be varied by the control voltage Vc of the control voltage generating source 104.
However, the thus formed variable gain circuit according to the related art has a circuit configuration with the differential circuits piled in two stages in the direction of the supply voltage. Therefore, supposing that the base-to-emitter voltage of the bipolar transistors is about 0.9 V at a maximum, and supposing that when the constant current source I101 is formed by a bipolar transistor, the collector-to-emitter voltage of the transistor is about 0.4 V, a voltage of about 2.2 V is required for operation of the differential circuits piled in two stages (differential amplifier circuit 101 and current divider circuit 102) and the constant current source I101.
Furthermore, when device variations and the like are taken into consideration, a supply voltage of at least about 2.5 V is required to prevent saturation of the differential circuits and the constant current source I101. In general, when the dynamic range and transient characteristics are taken into consideration, the related-art, variable gain circuit formed as described above is of a circuit type operated under a supply voltage of about 3.3 V. Thus, while there has recently been a tendency toward lower supply voltage in portable terminals, such as portable telephones and PDAs (Personal Digital Assistants), the related-art, variable gain circuit formed as described above cannot meet the need for lower supply voltage.
The present invention has been made in view of the above problem, and it is accordingly an object of the present invention to provide a variable gain circuit operable at a lower supply voltage.
According to an aspect of the present invention, there is provided a variable gain circuit including: a first transistor and a second transistor, each having a control electrode connected to a circuit input terminal; a load connected between a first power supply and a first electrode of at least one of the first transistor and the second transistor; a third transistor and a fourth transistor having second electrodes connected to the first transistor and the second transistor, respectively, and each having a first electrode and a control electrode connected to each other; a first variable current source connected between a second power supply and the second electrodes of the first transistor and the third transistor and having a current value variable according to an external control signal; a second variable current source connected between the second power supply and the second electrodes of the second transistor and the fourth transistor and having a current value variable according to the control signal; a current source connected between the first power supply and a node of the first electrodes and the control electrodes of the third transistor and the fourth transistor; and an impedance component having one end connected to the first electrodes and the control electrodes of the third transistor and the fourth transistor.
Both bipolar transistors and field-effect transistors can be used as the first to fourth transistors. In the case of using a bipolar transistor, the first electrode refers to a collector electrode reached by a carrier (electron or hole), the second electrode refers to an emitter electrode for injecting the carrier, and the control electrode refers to a base electrode supplied with a current for controlling movement of the carrier injected from the emitter electrode. On the other hand, in the case of using a field-effect transistor, the first electrode refers to a drain electrode reached by a carrier, the second electrode refers to a source electrode for supplying the carrier, and the control electrode refers to a gate electrode supplied with a signal for controlling the main current.